1. Field of the Invention
The present invention relates to a hydraulic drive system for a counterweight dolly in a counterbalance type crane.
2. Description of the Prior Art
Heretofore, as a large-sized travelling type crane there has been known a counterbalance type crane as described, for example, in U.S. Pat. No. 3,842,984 in which as shown in FIG. 5 a crane body is provided with a crawler 1, an upper swivelling structure 2, a main jib 3 and a mast 4, and a dolly 6 which carries a counterweight 5 thereon is connected through a connecting beam 7 to the crane body behind the upper swivelling structure 2, further the dolly 6 is connected to a guy line 8 suspended from an upper end of the mast 4 to thereby increase the lifting capacity and improve stability.
Practically applied is apparently a crane of this type in which wheels 9 disposed at the lower portion of the dolly 6 are provided with a drive unit for the travelling and swivelling of the counterweight dolly 6 following travelling of the crane body and swivelling of the upper swing member 2.
In this type of crane, as the lifting load increases, a large tension in a pulling-up direction for the dolly 6 acts on the guy line 8 which connects a fore end of the mast 4 and the dolly 6, and may result in lowering of the ground contact pressure of the wheels 9. In this case, the above conventional crane is not provided with means for detecting the ground contact pressure of the dolly wheels 9 and the driving force of a wheel driving motor, nor is it provided with means for adjusting those pressure and force. Consequently, even when the wheels 9 slip or skid with respect to the ground surface, the driving motor continues operation, resulting in a large quantity of energy being lost and the dolly 6 being prevented from travelling and swivelling smoothly. This is not only dangerous but there also is a fear of overload of a travelling drive system and of a swivelling drive system for the crane body. It is therefore necessary to either restrict the lifting capacity in a range capable of ensuring a ground contact pressure matching the driving force for the wheels 9, or make the counterweight heavier, resulting in the crane specification being limited. Thus various problems have plagued in conventional cranes.